The present invention relates to apparatus for providing thermal homogeneity of a gas medium, particularly for preventing optical distortion in pressurized gas Raman cells.
Although the present invention has particular application for use in pressurized gas Raman cells, it is to be understood that its applicability is as pertinent in apparatus where uneven heating of the gas results in optical distortion. Specifically, the Raman scattering process deposits heat into the pressurized gas medium in the form of vibrational excitation of the gas molecules by a laser beam. The vibrational energy, which is imparted to individual gas molecules, rapidly thermalizes causing localized heating of the gas. In a static (nonflowing) Raman gas medium, an optical distortion is produced along the beam path that persists for several hundreds of milliseconds following each laser pulse. Typically the optically distorted beam path limits the Raman laser pulse repetitive frequency (PRF) to a few hertz unless the gas is circulated.
Such gas circulation has been effected in the past by means of an internal motor and fan to increase the PRF. In such a circulating gas mechanization, a vaneaxial blower moves gas around a closed path within the cell. The gas is distributed along the focused laser beam axis and is moved transverse to the beam axis to remove the heated gas from the beam path between laser pulses. Gas plenums, connecting to the blower, distribute the gas along the beam axis. The maximum PRF is determined by the clearing time to remove the disturbed gas from the beam path.
Such circulation has successfully increased PRF capability, but at the cost of significantly increased size and complexity of the cell. In addition, the motor and fan introduce undesirable materials into the cell interior, and dissipate several times more heat into the gas than is produced by the Raman conversion itself Winding insulation materials and bearing lubrication required by the motor outgas and limit cell life when operated for long term as a closed system. The approximate 10 watts of unwanted heat that are dissipated into the gas by the motor also adds to the difficulty of maintaining a uniform gas temperature within the beam path.